Commonly known methods of producing thick steel rods, resistant to corrosion, with the cross-section surface area above 150 mm2 i.e. with the diameter of 14 mm, which are based on the expensive modification of the chemical composition of the steel followed by a plastic treatment such as e.g. forging do not permit achieving in these rods the tensile strength UTS above 1000 MPa and the yield stress YS above 900 MPa. There have also been known wires with high strength UTS>1000 MPa, but they have been produced by multiple-pass drawing, a technology which in common opinion cannot however yield thick rods.
The plastic treatment method called the hydrostatic extrusion has been known since over one hundred years (U.S. Pat. No. 524,504). In this method the billet (the material to be extruded) is placed in a high-pressure chamber filled with a pressure transmitting medium. The high-pressure chamber is closed from one side with a piston and from the other side with a die which is shaped adequately to the desired shape of the final product. When moving into the depth of the chamber, the piston compresses the pressure transmitting medium thereby increasing the hydrostatic pressure in the chamber. After the critical pressure, characteristic of the billet material, is reached, the billet begins to be extruded through the die to form the desired product. One of the important parameters of the hydrostatic extrusion process is what is known as the reduction R which describes the degree of reduction of the transverse cross-section of the billet and is defined as the ratio of the billet cross-section surface area before the extrusion to that of the product after the extrusion. Since the beginning of experiments with the hydrostatic extrusion process, there have been many literature reports describing the use of this method for treating various metals, alloys, composites, plastics, and other materials, but, on the industrial scale, it has never been used for hydrostatically extruding steel. The hydrostatic extrusion process was however investigated for experimental purposes and described by J. Budniak, M. Lewandowska, W. Pachla, M. Kulczyk, K. J. Kurzydtowski in “The influence of hydrostatic extrusion on the properties of austenitic stainless steel” [Solid State Phenomena 2006, Vol 114, pp 57-62]. The results reported in this publication concern rods with mechanical strength UTS>1200 MPa but with small diameters (below 6 mm). The rods were extruded using the cumulative method (a multi-pass process) with the reduction in one pass not exceeding 2. Neither was examined the effect of the hydrostatic extrusion of steel on the distribution of the mechanical properties on a transverse cross-section of the rods obtained. M. Pisarek, P. Kdzierzawski, T. Plociński, M. Janik-Czachor, K. J. Kurzydtowski in “Characterization of the Effects of Hydrostatic Extrusion on Grain Size, Surface composition and the Corrosion Resistance of Austenitic Stainless Steels” [Materials Characterization, 59, 9 (2009) 1292-1300] describe the results of their studies on the corrosion and other surface properties of hydrostatically extruded austenitic steel, but their experiments only included rods with small diameters, produced by the accumulation of several extrusion passes, each with a low cross-section reduction. The paper “Low-temperature mechanical properties of 316L type steel after hydrostatic extrusion” [Original Research Article Fusion Engineering and Design, Volume 86, Issues 9-11, October 2011, Pages 2517-2521] by P. Czarkowski, A. T. Krawczyńska, R. Slesiński, T. Brynk, J. Budniak, M. Lewandowska, K. J. Kurzydtowski presents the results of investigating the mechanical properties of austenitic steel subjected to hydrostatic extrusion at a low temperature, but this publication is only concerned with products of small diameters (up to 6 mm) produced in the cumulative way with a low one-pass reduction. In the available literature one cannot even find speculative opinions concerning the possibility of hydrostatic extrusion of steel conducted with a high reduction degree in one pass, or the possibility of using this technology with an arbitrarily high reduction degree, or else its use for the fabrication of steel rods with greater diameters.